MXene, protein, and KCl-assisted ionic conductive hydrogels with excellent anti-freezing capabilities, self-adhesive, ultra-stretchability, and remarkable mechanical properties for a high-performance wearable flexible sensor

Developing a hydrogel with switchable features and freeze tolerance is remarkably significant for designing flexible electronics to adjust various application needs. Herein, MXenes, AFPs (antifreeze proteins), and potassium chloride (KCl) were introduced to a polyacrylamide (PAM) polymer network to design an anti-freezing hydrogel. The ionic hydrogels are characterized by excellent ionic conductivity, presenting adjustable properties of remarkable mechanical strength and self-adhesion to meet individualized application demands. The capability of KCl and AFPs to inhibit ice crystals gives hydrogels with anti-icing properties under a low-temperature atmosphere. The PAM/MXene15/AFP30/KCl15 hydrogel demonstrated negligible hysteresis behavior, quick electromechanical response (0.10 s), and excellent sensitivity (gauge factor (GF) = 13.1 within the strain range of 1200–2000%). The resulting hydrogel could be immobilized on the animal or human skin to detect different body movements and physiological motions, offering reproducibility and precise accuracy as primary advantages. The approach of developing materials with tunable features, along with inorganic salt and the fish-inspired freeze-tolerance method, offers a new prospect for wearable gadgets.

times to neutrality with DI water.The obtained mixture was centrifuged with deionized water at 4500 rpm for 10 min, and the resulting supernatant was poured off.This phenomenon was repeated until the color of the supernatant turned black.MXene sediments were collected by applying a greater centrifugal speed of 7500 rpm.To prepare the MXene suspension, the previously collected sediments were introduced into 150 mL of deionized water and then sonicated in an Ar environment for 60 min, finally again centrifuged for 60 min at 3500 rpm.

Text S3. Adhesion test
The adhesive strengths of hydrogel with different substrates were determined using lap shear tests by employing the CT3-1000 texture analyzer (USA).The PAM/MXene/AFPs/KCl hydrogels were cut into 10 mm × 10 mm and placed between two identical substrates affixed to the plates.The adhesive strength of hydrogel was calculated by splitting the maximum load by the initial area of adhesion 1 .Glass, paper, silicone, metal, porcine skin, plastic, rubber, wood, ceramic, and PTFE were chosen as substrates to explore the universal adhesion property of hydrogels.

Text S4. Conductivity analysis
The electrochemical workstation (TH2831, made in China) was utilized to describe the conductivity of the hydrogel, which comprised differing concentrations of MXene, AFPs, and KCl.
The conductivity of hydrogel was calculated by the following formula: Where S (cm 2 ), R (Ω), and L (cm) denote the cross-sectional area, resistance, and test length of hydrogel, respectively.

Text S5. Mechanical test
The mechanical strength of PAM/MXene/AFPs/KCl hydrogels was examined by a multifunctional testing machine with a 100 N load sensor (Shimadzu AGS-X, Japan).The PAM/MXene/AFPs/KCl hydrogels were prepared in dumbbell-like shape with thickness of 3 mm and width of 4 mm.

Text S6. DSC analysis
A differential scanning calorimeter (DSC25, TA, made in the USA) was used to investigate the freeze resistance of PAM/MXene/AFPs/KCl hydrogel at low temperatures.The DSC analysis started at a temperature of 20 °C and then decreased to -60 °C.

Text S7. Electromechanical measurements
The electrochemical workstation (Vertex C, IVIUM Tech, made in the Netherlands) connected to the tensile tester (MCT-2150, A&D, made in Japan) was employed to investigate the electromechanical response of the hydrogels.The electrochemical workstation recorded output signals as hydrogel was stretched by using a tensile tester.However, the PAM/MXene/AFPs/KCl hydrogels were immobilized on the human body to serve as hydrogel-based sensors to monitor bodily motions by attaching them to metal wires.The equation given below was employed to record the output signal.
where R 0 and R denote resistance before and after applying strain on the PAM/MXene/AFPs/KCl hydrogel.

Figure S8 :
Figure S8: Effect of different MXene contents on the conductivity of hydrogels (a), and Effect of different KCl contents on the conductivity of hydrogels.

Figure S9 :Figure S10 :
Figure S9: relative resistance changes of hydrogel-based sensors as the volunteer touched the iron hard ball or soft yarn ball (a) and captured various weights (b)

Figure S10 :
Figure S10: Electrical output signals of PAM/MXene 15/ AFP 30 /KCl 15 hydrogel-based sensors for detection of tail swinging of mouse (a), ΔR/R 0 curves versus time of PAM/MXene 15/ AFP 30 /KCl 15 hydrogel-based sensor affixed on a mouse leg for monitoring free movement (b), respiratory output signals from the PAM/MXene 15/ AFP 30 /KCl 15 hydrogel-based sensor under the painful and normal condition (c)